A rear loading refuse truck of the type disclosed, for examples, in U.S. Pat. Nos. 3,767,068, 3,777,917 and 4,029,224, usually has an ejection panel mounted within the body of the truck which is powered to move back and forth as necessary, first to act as a bulkhead against which refuse can be packed and compacted and secondly to act as an ejector for pushing the refuse out of the body of the truck after it is loaded. In such a truck a rear tail gate is provided which has a refuse accumulation hopper at its bottom and a packer blade which is cycled to move through a closed path from a position in which the packer blade closes off the rear of the body, backwardly over accumulated refuse in the hopper and downwardly through the hopper to carry the refuse forwardly out of the hopper and to pack it into the rear of the body, compressing it as much as possible against the ejection panel.
Most vehicles of this type have hydraulic systems including hydraulic cylinders and piston rods which actuate the packer blade. The cylinders are controlled by sequentially operated spool valves which are provided with pressure responsive release means to deactivate them to neutral if the pressure in the connected lines reaches a predetermined level. In order to obviate possible malfunction resulting from shock pressure, pressure surges and the like, the pressure responsive relief means are set at levels lower than maximum system pressure. For example, with a maximum system pressure of 2000 p.s.i., the pressure responsive means might be set at 1700-1800 p.s.i. and if there is a second valve in a "down stream" connection, it should be set 200-300 p.s.i. lower.
Because of the necessity of building into such a system a pressure relief means, if the pressure exerted by the refuse trapped between the packer blade and the ejection panel reaches a level of 1700 p.s.i., the pressure responsive mechanism functions and the packer blade stops. Sometimes this position is reached before the packer blade has returned to its rest position closing the rear of the body and often the operator will over-control the pressure responsive mechanism, as by holding a feeding valve closed, in an effort to apply to the refuse the full 2000 p.s.i. In many cases, this application of the full pressure delivered by the pump to the system is sufficient to carry the packer blade to its rest position thus further compacting the refuse in the space between it and the ejection panel. However, if this extra manual operation is required the operator must remain at the truck. This means that that the operator cannot otherwise be getting containers of refuse and dumping them into the refuse hopper.
It is, therefore, the principal object of the instant invention to provide an hydraulic apparatus for a rear loading refuse truck, or the like, which is not only provided with a pressure responsive relief system for ordinary protection of the system but also has what might be called a "system hold" mechanism which is responsive to the position of the packer blade relative to the loading hopper and the truck body and not merely to the pressure created in the system.
It is yet another object of the instant invention to provide an hydraulic system for a rear loading refuse truck, or the like, in which the control valves for the packing cycle of the packer blade are held in power position during the movement of the packer blade through the hopper and up toward the ejection panel in the truck body until the packer blade reaches a position very close to but slightly short of its final rest position so as to apply to the blade during the packing stroke the full power potential of the hydraulic system even through it may momentarily pass the usual pressure responsive cut off.